

1894.] On the Electrification of Air. 93 



where V denotes the potential indicated by the water- dropper, a the 

 radius of the spherical hollow, and p the electric density of the air at 

 distance r from the centre. Supposing now, for example, / to be 

 constant from the surface to the centre (which may be nearly the case 

 after long electrification as performed in our experiments), we find 

 V = f Trpa? ; whence p = 3V/2?ra 2 . 



To particularise further, suppose the potential to have been 

 38 volts or O127 electrostatic c.g.s. (which is less than the greatest 

 found in our experiments) and take a = 50 cm. : we find p = 2'4.10~ 5 . 

 The electrostatic force at distance r from the centre, being |- TT pr, is 

 therefore equal to 10~*r. Hence a small body electrified with a 

 quantity of electricity equal to that possessed by a cubic centimetre 

 of the air, and placed midway (r = 25) between the surface and 

 centre of the enclosure experiences a force equal to 2'4.10~ 9 .25, or 

 6.1CT 9 , or approximately 6.10~ s grammes weight. This is 4'8 per cent. 

 of the force of gravity on a cubic centimetre of air of density 1/800. 



14. Hence we see that, on the supposition of electric density 

 uniform throughout the spherical enclosure, each cubic centimetre of 

 air experiences an electrostatic force towards the boundary in simple 

 proportion to distance from the centre, and amounting at the bound- 

 ary to nearly 10 per cent, of the force of gravity upon it ; and electric 

 forces of not very dissimilar magnitudes must have acted on the air 

 electrified as it actually was in the non-spherical enclosure used in 

 our experiments. If natural air or cloud, close to the ground or in 

 the lower regions of the earth's atmosphere, is ever, as in all pro- 



ibility it often is, electrified to as great a degree of electric density 

 as we have found it within our experimental vat, the natural electro- 

 static force in the atmosphere, due as it is, no doubt, to positive elec- 

 tricity in very high regions, must exercise an important pondero- 

 motive force quite comparable in magnitude with that due to 

 difference of temperatures in different positions. 



It is interesting to remark that negatively electrified air over 

 negatively electrified ground, and with non-electrified air above it,. 

 in an absolute calm, would be in unstable equilibrium ; and the nega- 

 bively electrified air would therefore rise, probably in large masses, 

 through the non-electrified air up to the higher regions, where the 

 positive electrification is supposed to reside. Even with no stronger 

 electi-ification than that which we have had within our experimental 

 vat, the moving forces would be sufficient to produce instability com- 

 parable with that of air warmed by the ground and rising through 

 colder air above. 



15. During a thunderstorm the electrification of air, or of air 



id the watery spherules constituting cloud, need not be enormously 

 stronger than that found in our experiments. This we see by con- 

 sidering that if a uniformly electrified globe of a metre diameter 



